Is there any sage gear for a level 15 that has pants I stead of skirts? Just wondering. Also, once you get moddable gear do you lose anything if you keep it and just keep adding new mods? Is there a base stat on it that later gear has higher stats for?
As mentioned before, the Social Vendor on Coruscant sells the "Formal" set. All the pieces are empty shells and can be modded fully. The "Formal" set requires Social 1 to equip (a few runs of Esseles or Hammer Station with the Group Finder). They are however "male only" so if your Sage is female this won't work.
For female characters, there is the Balmorra "Social Set" called "Balmorra Resistance". It also has a pair of trousers. These are "unisex" and require Social 2 (also pretty easy to get). As Republic you would only normally go to Balmorra much later as party of your story. However, once you have your ship you can travel there at any time and the Social Vendor is in the Cantina of where you land on the planet. So easy to get. NOTE: The trousers from the "Balmorra Resistance" set might not color match too well with the chest piece you are using.
Finally, in terms of easy to acquire trousers for Sages, you have the 2 trousers that can be purchased from the Alderaan Equipment Commendations vendor. They are the "Enshrouding Force Legwraps" and the "Muse Lower Robe" (don't be fooled by the names, they are trousers) and they cost 12 commendations and require level 29 (if you go the commendations vendors on the fleet you can preview them). Both of these trousers will sometimes crop up on the GTN in their [Prototype] ("purple" quality) variation and in this case they require level 28 to equip (HOWEVER they are normally VERY expensive on the GTN).